It is worth mentioning that a single plane wave is obliged to exert positive radiation pressure on any passive system in the paraxial approximation due to momentum conservation [1]}, [2]}, [3]}. For example, the optical force \(F\)  acting on a slab located in vacuum can be expressed as \(F =N_i\hbar k {(1 + r - t)}\) , where \(N_i = c \varepsilon _0 E_0^2/2\hbar \omega \)  is the photon density, \(E_0^2\)  is the electric field intensity, c is the speed of light, \(\omega \)  is the angular frequency of the light and \(\varepsilon _0\)  is the vacuum permittivity. Furthermore, \(t \)  and \(r\)  are the Fresnel transmission and reflection coefficients that use to define the momentum transmitted and reflected through the slab, respectively. In this regard, the optical force exerted on a perfect reflector  (\( r = 1, \, t = 0\) ), black body (\( { r} = 0,\, t = 0\) ), and perfect transmitter (\( r = 0, \, t = 1\) ) can be estimated as \(F = 2 N_i\hbar k\) , \(F = N_i\hbar k \)  and \(F = 0 \) , respectively. Thus, the net force can not be negative due to fact that the transmitted or reflected photons cannot have larger momentum than the incident ones and hence, \(F\ge 0\)  [1]}, [5]}, [6]}, [7]}.
